Oral anticoagulants (OA) are a leading cause of fatal haemorrhagic adverse events in relation with an important interindividual variability of response to these drugs. Besides several clinical factors, this interindividual variability of response to OA has a pharmacogenetic basis. Carriers of cytochrome P450 2C9 (CYP2C9)-deficient alleles have a reduced clearance of warfarin and are exposed to dramatic overdoses in the first weeks of treatment. Genetic polymorphisms of vitamin K epoxide reductase (VKORC1), the target of OA, identify patients with a high sensitivity to OA who are at risk of early overdose. Most pharmacogenetic evidence is presently restricted to warfarin. Several warfarin dosing algorithms have been constructed, adapted on CYP2C9 and VKORC1 genotypes and clinical factors, to predict the best dose for each patient. Carriers of one of allelic variant need a 20-30% reduction of warfarin dose. However, definite evidence concerning the usefulness of these algorithms in terms of reducing the frequency of major bleeding episodes is still lacking. Ongoing prospective randomised trials will ascertain definitive answer over the coming years.
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